A fuel cell is an electrochemical device to convert chemical energy of fuel gas such as hydrogen gas and oxidizer gas containing oxygen electrochemically into electric energy. A typical fuel cell has an electrolyte membrane in contact with an anode and a cathode on either side. Fuel gas is continuously fed to the anode and the oxidizer gas is continuously fed to the cathode. The electrochemical reactions take place at the electrodes to produce an electric current through the electrolyte membrane energy, while supplying a complementally electric current to the load.
Load to a fuel cell used in a vehicle application as a drive power supply rapidly changes in a wide range from idling to full acceleration. In order to prevent insufficient supply of reactant gas at any site in fuel cell stack, fuel gas and oxidant gas are supplied to the fuel cell at a mass flow rate more than a mass flow rate necessary to produce the required output current. A ratio of mass flow rate of the supplied reactant gas to a mass flow rate of reactant gas theoretically required for producing the output current is referred to as a gas supply excess rate, or a stoichiometric ratio (SR). Operation under excessive stoichiometric ratio results in an increased power consumption of auxiliary equipment such as an air compressor for supplying air as oxidant gas, lowering fuel efficiency of the fuel cell system.
In the case that load to the fuel cell increases and the supply of reactant gas becomes insufficient for the requested output current, current may concentrate at local areas inside the fuel cell stack, causing a local temperature increase and lowering output voltage of the fuel cell.
Japanese Patent Application Laid-Open Publication No. 10-326625 discloses a device for limiting output current of a fuel cell so that the output current does not exceed the amount corresponding to reaction gas supply rate, when load to the fuel cell drastically increases, and thereby supplying the reaction gas at a rate determined based on the output current thus limited.